The invention relates to a method for the variable rotational speed control of a displacement pump unit for delivering a fluid, comprising a displacement pump and a drive, the drive comprising an electric drive motor provided with stator and rotor and a frequency converter, by a control device, a controlled variable of the displacement pump being controlled to a set point by the control device, and to a suitable control device for implementing the method and a corresponding displacement pump arrangement.
Displacement pumps are frequently used to deliver fluids, i.e., liquids or gases, at medium to high pressures and with small delivery rates. The drive usually employed is an asynchronous electric motor with upstream frequency converter in combination with a control device, formed as a process controller, for the variable rotational speed operation of the asynchronous motor. By the variable rotational speed operation together with a measurement of the final pressure as controlled variable of the displacement pump, control of the pressure at a variable delivery rate can be achieved. In addition to the actual control, the control device is usually additionally provided for open-loop control, monitoring and/or diagnostic tasks. The frequency converter and control device are usually implemented separately. In order to avoid overpressures and to dissipate pressures quickly, separate pressure relief valves are used on the pressure side in a conventional displacement pump arrangement. Because of the drive, starting up the drive against a closed slide valve is not possible without control actions by the pressure relief valve. Maintaining a required pressure at a delivery rate equal to zero, what is known as a zero delivery rate, is not possible in a conventional arrangement. As a result, the time needed to adjust the pressure as the slide valve is opened is prolonged.
Furthermore, a method is known for the sensorless control of rotor angular position or rotor position, called the position below for simplicity, and the rotational speed of a synchronous reluctance motor. Available as state values are rotational speed and position of the synchronous reluctance motor and, via the torque-forming current component, the torque of said motor.
The object of the invention is to devise a method for the variable rotational speed control of a displacement pump unit which increases the dynamics of the control and requires fewer individual components of the arrangement, and to provide a suitable control device for implementing the method and a corresponding displacement pump arrangement.
According to the invention, this object is achieved by a method in which, by the control device, at least one state value made available by the drive is registered, in particular torque and/or position of the drive motor, and, from this, the controlled variable of the displacement pump is determined by the control device, in order to control the displacement pump unit to the set point without the use of sensors. In particular, the final pressure and the required fluid volume are provided as controlled variables of the displacement pump. Advantageously, torque and position of the drive motor are registered as state values of the drive. In the method according to the invention, control to a set point is devised which manages entirely without any measurement of the controlled variable by sensors and is based purely on the state values supplied by the drive.
Furthermore, the control device is provided for open-loop control, monitoring and/or diagnostic tasks. In this application, without restricting the overall functionality of the device, the term control device will be used for simplicity. According to the invention, frequency converter and control device can also be implemented in an integrated manner.
One refinement of the method provides for the final pressure to be determined by the control device as controlled variable of the displacement pump, in order to control the displacement pump unit to the final pressure set point without using sensors. As a result, control to a final pressure set point is devised which manages without measurement of the final pressure by sensors.
According to a refinement of the invention, provision is made for the final pressure of the displacement pump to be controlled by a motor torque-final pressure dependency of the displacement pump unit that is stored in the control device. The control device and/or the drive implemented in an integrated manner with the control device is configured or matched to the respective displacement pump by such a pressure model. The final pressure of the pump is controlled via the control of the motor torque. The motor torque-final pressure dependency stored in the control device is provided in the form of a characteristic curve, a table of values or the like. In addition, a relationship in the form of a formula is provided and can be stored in a memory device provided in the control device. A simple linear relationship between motor torque and final pressure has proven to be expedient and adequate to a first approximation, said relationship being given by the actual value of the final pressure pact, the actual value of the motor torque Mact, and the constant k1 through the following equation:Pact=k1·Mact  (Equation 1)
For a precise determination of the final pressure, it has proven worthwhile that the rotational speed of the motor can additionally be registered by the control device. By such a dynamic pressure module, for the starting operation, the determination of pressure can be carried out while taking the dynamic torque component into account, formed from the product of the motor inertial constant θ and the derivative of the rotational speed ωact, in accordance with the following equation:Pact=k1·(Mact−θ·ωact′)  (Equation 2)
Moreover, it has proven to be expedient to register state values, in particular position, rotational speed and torque, of a sensorless synchronous reluctance motor with flux barrier gap by the control device. The rotor angular position, also called position below, and rotational speed of a rotor of a synchronous reluctance motor that is provided with flux barriers or cutouts called flux barriers in the rotor lamination can be controlled without sensors. In addition to the state values comprising rotational speed and position of the synchronous reluctance motor, the torque of the motor with the torque-forming current component is also available. The rated torque of the sensorless synchronous reluctance motor is already available when the motor is at a standstill, so that, even in the case of a delivery rate of zero, the pressure can be kept at the required level. The method according to the invention in conjunction with the sensorless synchronous reluctance motor permits a displacement pump to be started up against a closed slide valve. The desired final pressure is immediately available in this case.
It is particularly advantageous that such a synchronous reluctance motor is controlled without rotational speed sensors and without position sensors. Thus, by using the method according to the invention, there is control to a final pressure set point which manages without any measurement of the final pressure by sensors and without any position or rotational speed sensors.
Additional advantages are provided by a refinement of the method according to which the position of the drive motor, that is to say the angular position of the drive rotor in relation to the drive stator, is registered by the control device and, by using the value of the sealed displacement pump volume, the stroke volume, the delivered fluid volume is determined. Here, the delivered fluid volume is given by the angular distance covered by the drive rotor and the stroke volume of the displacement pump. Thus, defined fluid volumes may be delivered by the method. Final pressure and delivered fluid volume of the displacement pump unit can thus be determined simultaneously.
Furthermore, by the combination of final pressure control and determination of the delivered fluid volume, incomplete filling of the displacement pump with the fluid to be delivered can be detected. As a result of detecting an incomplete filling that may possibly be present, a correct calculation of the delivered quantity can be carried out.
Provision is made here for a predefined fluid volume to be delivered the actual fluid volume determined being compared by the control device with the predefined fluid volume and, when the predefined fluid volume is reached, delivery operation of the displacement pump unit being stopped. To this end, for example beginning with a starting time, the delivery volume per piston stroke of the displacement pump is added up. When the predefined fluid volume is reached, the delivery operation of the displacement pump unit is stopped. To this end, a valve arranged on the pressure side can be activated by the control device and can be closed.
In a control device according to the invention for implementing the method according to the invention, a controlled variable of a displacement pump being controlled to a set point by the control device, provision is made for the control device to register at least one state value provided by the drive, in particular torque and/or position of the drive motor, and to have a memory device, for the control device to determine the controlled variable from the state value, in order to control the displacement pump unit to the predefined set point without using sensors and on the basis of the state values supplied by the drive. To this end, torque and position of the drive motor are expediently registered as state values of the drive. In particular, provision is made for the final pressure of the displacement pump to be used as controlled variable. This permits the control of the final pressure to the final pressure set point without pressure sensors. From the position information from the drive motor, it is possible to control to the controlled variable of delivered fluid volume.
According to an advantageous refinement, provision is made for a motor torque-final pressure dependency of the displacement pump unit to be stored in the memory device of the control device and for the control device to control the final pressure of the displacement pump by the motor torque-final pressure dependency of the displacement pump unit. The memory device is used to store the characteristic parameters or characteristic variables of the respective displacement pump. This is carried out in the form of a characteristic curve, a table of values, by a relationship in the form of a formula or the like. Expediently, a linear relationship between motor torque and final pressure in accordance with the above equation 1 is stored in the memory device.
Furthermore, the control device according to the invention is capable of using the position information from the drive motor, more precisely the angular position of the drive rotor, and the value of the sealed displacement pump volume, the stroke volume, to determine the fluid volume delivered and/or to control a predefined fluid delivery rate.
A displacement pump arrangement according to the invention, having a displacement pump unit for delivering a fluid, the displacement pump unit comprising a displacement pump and a variable rotational speed drive, the drive comprising an electric drive motor and a frequency converter, a controlled variable of the displacement pump, in particular final pressure and/or delivered fluid volume, being controlled, and possibly having a valve arranged on the pressure side, in particular a shut-off valve, is characterized by a control device according to the invention. If appropriate, the valve, in particular the shut-off valve, is actuated and/or controlled by the control device.
Advantageously, the drive motor of the displacement pump unit is a synchronous reluctance motor having flux barriers and operated without sensors. The rotor angular position and rotational speed of a synchronous reluctance motor that is provided with flux barriers can be controlled without sensors. Rotational speed, position and torque of the synchronous reluctance motor are available to the control device as state values. The rated torque of the sensorless synchronous reluctance motor is already available when the motor is at a standstill, so that, even with a delivery rate of zero, the pressure can be kept at the required level. By using the sensorless synchronous reluctance motor, even starting up the displacement pump unit against a closed slide valve is possible. The desired final pressure is immediately available in this case.
Here, it has proven to be expedient that the drive determines the position and the rotational speed of the drive motor without sensors. To this end, the drive measures electric voltages and/or electric currents of the drive motor.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.